The present invention relates to an electron beam depicting technique and, particular to a technique for applying electron beams to form a predetermined pattern, for example, a diffraction pattern corresponding to an optical element, on a substrate as an exposure target.
In recent years, a CD and DVD, for example, are widely used as an information-recording medium, and many optical elements are used in the precision apparatus such as reading apparatus for reading such a recording medium. To ensure reduced costs and compact configuration, a plastic optical lens, rather than a glass-made lens, is more often used as the optical element such as an optical lens employed in this apparatus.
The aforementioned plastic optical lens is manufactured by general injection molding, and the metallic molds used for injection molding operation are formed by general cutting operations.
In recent, years, the required level of specifications and performances of the optical element has been raised. For example, when manufacturing the optical element having a diffraction structure on the optically functioning surface, the metallic mold must be provided with a surface for forming a diffraction structure in order to carry out injection molding of the optical element.
However, if an attempt is made to form a fine configuration such as a diffraction structure on a metallic mold by a cutting tool employed in the molding technique or working technique currently employed, working accuracy is deteriorated and the strength and service life of the cutting tool are reduced. Thus, precision working on the order of submicrons or even less has been difficult so far.
Especially in the pickup lens used in the medium such as a DVD, as compared with the CD-ROM pickup lens, diffraction structure of higher precision is required to cope with an increase in the recording density. The required working accuracy is on the level smaller than the light wavelength, for example, on the order of nm. The prior art working method, however, has failed to meet such high-precision working requirements.
In the meantime, to form a predetermined pattern on the surface of a substrate including an optical element, an optical aligning method, for example, a method using an aligner based on mask exposure technique has been employed.
By way of an example, an aligner for patterning a predetermined shape on the surface of a substrate such as the wafer substrate (photo mask) of a semiconductor may be used for patterning on the surface of the aforementioned optical element or for machining of a metallic mold. However, the apparatus for wafer substrate has a problem in that only a flat material can processed. Further, in the apparatus for wafer substrate, the working depth of the substrate is controlled in terms of exposure energy. However, in the case of precision working of a diffraction grating for optical elements and others or creation of a photonic crystal, the structure shorter than the wavelength of the applied light must be accurately formed on a non-planar surface such as a lens. Thus, an aligner according to the aforementioned control method is not suitable for fine patterning of the required level.
A laser beam method is another candidate for consideration. Laser beam is sometimes used for working on the order of microns, but the beam diameter is optically controlled, and beam convergence is limited. In this sense, this method is not suited for working on the order of submicrons, especially on the level close to the wavelength of light. Further, since a sufficient depth of focus cannot be obtained, it is necessary to utilize such a mechanical means as auto focusing at all times. This is one of the causes for discouraging high-precision working. Especially when high precision is required in the patterning of an optical element having a curved surface (including a 3D configuration having a surface changing on a macroscopic scale), this problem becomes serious.
Thus, this art can be employed in the patterning of a substrate having a planar form, but is not suited for forming a fine pattern on the substrate having a dynamic 3D configuration such as a curve. This has created a problem in this art.
To manufacture a metallic mold for such an optical lens, an attempt has been made to use an electron beam depicting apparatus to form a diffraction structure on the optical function surface of the optical element serving as the mother die. (See the Patent Document 1, for example).
In the aforementioned electron beam depicting apparatus, electron beam is applied to the surface of a substrate as a model of the optical lens. A predetermined dose of beam is used to perform scanning operation within a predetermined, whereby binary or blaze-shaped diffraction structure is formed.
[Patent Document 1]                Tokkai 20002-333722 (Japanese Laid Open Patent Publication)        
Incidentally, the minimum resolution of dose (hereinafter referred to as “minimum dose resolution”) is determined by the minimum time resolution of the analog-to-digital converter of the electron beam depicting apparatus. Therefore, dose adjustment is made in a conspicuously stepwise manner, especially when patterning is to be performed at a high current value within a limited time. Thus, even if efforts are made to form a smooth sloped blaze surface in the formation of a blaze-shaped diffraction structure, the sloped surface of the blaze obtained after development of the substrate becomes stepwise in shape, due to the magnitude of the aforementioned minimum dose resolution. This has created a problem in this art.
If the sloped surface of the blaze becomes stepwise in shape as described above, the optical characteristics of the optical lens are deteriorated; with the result that diffraction efficiency is reduced in particular. Further, when the consideration is given to the product quality, the aforementioned disadvantage will cause product value to be reduced. To improve the diffraction efficiency of the optical lens and to enhance product value, the sloped surface of the blaze must be made as smooth as possible in shape.
To solve such problems, it would be possible to perform patterning using a large beam diameter (at the defocusing position of electron beam, for example), thereby getting a smooth shape of the sloped surface of the blaze obtained after development of the substrate. In this case, however, a problem arises in that the rising shape becomes less sharp in the edge portion of the blaze.
When the rising shape becomes less sharp in the edge portion of the blaze as described above, the optical characteristics of the optical lens are degraded, and the diffraction efficiency in particular is deteriorated. Further, when the consideration is given to the product quality, the aforementioned disadvantage will cause product value to be reduced. To improve the diffraction efficiency of the optical lens and to enhance product value, the rising shape in the edge portion of the blaze must be made as sharp as possible.
In the meantime, the electron beam applied in the form of having a large beam diameter has the disadvantage of being seriously subjected to external disturbances. Further, in the commonly employed electron beam depicting apparatus based on spot beam method and others, the diameter of electron beam is set within the range from several nanometers to several tens of nanometers. If the diameter of electron beam is made larger, aberration or other defect will occur, and practical utility will be reduced.
To solve such a problem, the following technique can be considered: When the dose of electron beam is changed from the first dose to the second dose (where the difference between the first and second doses is equivalent to the dose of minimum adjustment unit based on the analog-to-digital converter minimum clock) while using a small-diameter beam for the purpose of patterning the sloped surface of the blaze, a sloped portion of different doses is provided, where the portion formed by both the first and second doses is present between the portion of the substrate formed by the first dose and that formed by the second dose. After development, the sloped portion of different doses is formed to have a height intermediate between the height of the portion of the substrate formed by the first dose and that of the portion formed by the second dose, with the result that the sloped portion of the blaze is made smooth and the rising portion of the edge is made sharp.
However, as described above, this is accompanied by the problem of requiring a great amount of time if both the sloped portion and edge of the blaze and the edge—the entire blaze—are formed using a small-diameter beam.
In order to get a smooth sloped portion of the blaze, it is necessary in practice to use a large-diagram beam. By contrast, a small-diameter beam must be used to get a sharp rising portion of the blaze edge. The prior art, however, has failed to get a smooth sloped portion of the blaze and a sharp rising portion of the blaze edge and to reduce the overall blaze patterning time, at the same time.